Data storage devices have methods to ensure the integrity of the data. In one method, called "read-after-write," data is put onto the media, and is then immediately read to ensure that the data was correctly written. There is also a process that separately and non-automatically looks at the media and checks to see that it can be read back from the device. This disc verification is a utility (called ScanDisk) that can be used on current PCs.
The mechanism of read-after-write is disadvantageous in a slow device in that it takes a significant amount of extra time to run that process. For example, a CD has a much slower read and write speed than does a hard drive. The data rates for a CD for writing are something today between 300 and 600 Kbytes per second, and a hard drive is on the order of ten times faster than that. The reading speed of a CD is about six times faster than the writing speed, and a hard drive reading speed is the same as the writing speed. Thus, if the user needs to conduct an operation on a CD where data is written and read back, it will be significantly slower than if the user were able to simply write the data to the disc without verifying it. This is how the file system for CD currently works. Data is written but not verified.
When the user does a format operation to prepare a CD disc for taking data, the system typically writes to that disc to arrange where the data will ultimately be put and then verifies that the data is able to be read back. But after the data has been written, the user has no assurance that something hasn't happened to damage the disc between the time of format and the time of writing data. Any rewritable particular media will be used over a long period of time, and many things can happen to that disc. The disc can be scratched, get fingerprints and dirt on it, etc.--All of these factors will affect both the successful writing of additional data as well as reading of the data.
On a hard disk, the media is in a very controlled, sealed environment. You cannot put a fingerprint on a hard disk. You cannot get dirt on a hard disk. But a removable media is exposed to the environment, thereby making it more vulnerable to damage. CD media is particularly susceptible to this damage because it is the only rewritable media that does not have a protective case (i.e. audio tapes, VCR tapes, floppy disks, etc.) When a user does the formatting and verifying, the disc is physically secure since it is in a device away from fingers, etc. But when the CD is removed from the drive problems arise. This is a significant difference between hard disk and CD technology.
All data storage devices take advantage of error correction designed in the format. There are many error correction algorithms. They are all effective in correcting errors and defects, but every method has limitations. A thick thumb print on a CD disc, for instance, will render sections of the CD disc unreadable. Since this type of reusable media is in a very unprotected environment, it will be handled and is likely to become soiled and damaged at random times over a long period of time. Therefore, a method is needed to ensure the steadfastness of data integrity.